A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy. A fuel cell generally consists of two electrodes positioned on opposites of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat.
The automotive industry has turned to fuel cells, particularly solid oxide fuel cells (SOFCs), to help power automobiles and reduce emissions. SOFCs are constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. A conventional electrochemical cell in a SOFC is comprised of an anode and a cathode with an electrolyte disposed therebetween. In a typical SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to the external circuit, and mostly water and carbon dioxide are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
The SOFC cell stack also includes conduits or manifolds to allow passage of the fuel and oxidant into and byproducts, as well as excess fuel and oxidant, out of the stack. Generally, oxidant is fed to the structure from a manifold located on one side of the stack, while fuel is provided from a manifold located on an adjacent side of the stack. The fuel and oxidant are generally pumped through the manifolds and introduced to a flow field disposed adjacent to the appropriate electrode. The flow fields that direct the fuel and oxidant to the respective electrodes typically create oxidant and fuel flows across the electrodes that are perpendicular to one another.
Seals must be provided around the edges of the various cell stack components to inhibit crossover of fuel and/or oxidant. For example, seals are disposed between the electrodes and adjacent flow fields, around manifolds, between flow fields and cell separators, and elsewhere. One factor in establishing SOFC reliability is the integrity of these seals.
Leaks in the manifold seals, electrochemical seals, or other defects can lead to the SOFC failure. When the concentration of oxygen on the anode side forms an oxidizing environment, anode oxidation can occur, creating a chemical and volume change that results in mechanical failure of the SOFC cell. To address this problem, conventional SOFC systems provide a continuous supply of fuel (or reformats) to continue to provide hydrogen to the anode (i.e., maintaining a reducing environment) and inhibit anode oxidation. However, during the period of shut down and start up, generally the reformate is not present in the fuel cell and the concentration of oxygen can be elevated, causing anode oxidation.
The drawbacks and disadvantages of the prior art are overcome by the use of strategies for anode protection.
A method of preventing anode oxidation in a fuel cell is disclosed. The method comprises applying a negative current to an anode of the fuel cell, such that the anode is disposed in ionic communication with a cathode through an electrolyte. Oxygen is transferred from the anode through the electrolyte to the cathode.
A method of preventing anode oxidation in a fuel cell is disclosed. The method comprises doping an anode of the fuel cell with a dopant, such that the anode is disposed in ionic communication with a cathode through an electrolyte. The dopant scavenges at least a portion of the oxygen present at the anode.
A method of preventing anode oxidation of a fuel cell is disclosed. The method comprises storing a reformate in fluid communication with the fuel cell having an anode and a cathode disposed on opposite sides of an electrolyte. The reformate is stored with materials selected from the group consisting of hydrides, carbon nano-tubes, and combinations comprising at least one of the foregoing materials. The reformate is introduced to the anode when the fuel cell has a temperature of about 400xc2x0 C. to about 1,000xc2x0 C.
The above described and other features are exemplified by the following figures and detailed description.